Steels and method for production of same

ABSTRACT

Improved steels and method for the manufacture of same, wherein an elongated steel workpiece is subjected to the steps of cold drawing to pre-strengthen the steel, straightening the workpiece and then rapidly heating the workpiece to a temperature within the range of 500*F to the lower critical temperature of the steel in less than 10 minutes. Steels produced in accordance with the present invention have high tensile strength, high yield strength and low residual stresses.

United States Patent Jones et a1.

[11] 3,908,431 [4 1 Sept. 30, 1975 15 1 STEELS AND METHOD FOR PRODUCTIONOF SAME Inventors: Samuel H. Jones, Flossmoor; James M. McNeany, ParkForest South,

both of Ill.

Assignee: LaSalle Steel Company, Chicago, 111.

Filed: May 7, 1974 Appl. No.: 467,639

US. Cl. 72/364; 148/12 B Int. Cl. B21C 9/00 Field of Search 148/12 B;72/364 [56] References Cited UNlTED STATES PATENTS 1,928,727 10/1933Johnson 148/12 PROPERTY Young .l 148/12 Behar 148/12 PrimaryE.\'aminerLowell A. Larson ABSTRACT 20 Claims, 7 Drawing Figures TENS LESTRENGTH YIELD STRENGTH RES/DUAL 5mm LfVEL 1515c. 1min, ZITOQR RES/STANCE 1.06 TIME kl FURNACE U.S. Patent Sept. 30,1975

Sheet 1 0f 2 $2 PM, I 2} AFTER DRAWING 7) I 55/ 0/25 STRAIGHTEN/NG) I I3 I AFTER STRAIGHTEN/NG) II 63 I I I I I I I I I I I l STRAIN U.S.Patent Sept. 30,1975 Sheet 2 of 2' 3,908,431

FIG 2 RES/DUAL STRESS LEVEL lszc. 1mm, InouR 106 TIME RESISTANCE FURNACESTEELS AND METHOD FOR PRODUCTION OF SAME This invention relates topre-strengthened, stressrelieved elongated steel products, and moreparticularly to steels having high tensile strength, high yield strengthand a low level of residual stresses.

As is well known to those skilled in the art, prestrengthened coldfinished steel bars are characterized by a combination of high strengthwith good machinability and thus, usable parts can be manufactured fromthe steel bars by various machining operations without the need forsubsequent heat treatment steps.

Up to the present, pre-strengthened cold finished steel bars or rodswere generally produced by cold drawing a bar or rod through a die whileeffecting a relatively large reduction in the cross-sectional area ofthe bar or rod. The cold drawing operation is then followed by astraightening operation in which the bar or rod is passed through aplurality of rollers which successively bend the bar or rod throughdecreasing arcs to effect a straightening of the bar or rod.Straightening apparatus for this purpose are well known to those skilledin the art and include Lewis straightening equipment and Medartstraightening equipment. It has been found desirable to follow thestraightening operation by a stress relieving operation to provide ahigher level of yield strength in the bar or rod as well as a somewhatreduced level of residual stresses.

The stress relieving operation is usually carried out in a batch orcontinuous furnace maintained at a temperature within the range of 500to 1200F. Uniformity, from the standpoint of uniformity of a single barand uniformity from bar to bar processed in a furnace is an importantconsideration in maximizing the yield strength while minimizing'theresidual stress level. It has been found that the uniformity of theresulting bar or rod is directly related to the degree of temperatureuniformity maintained during the heating process. Production furnacesused in the commercial manufacture of stress relieved bars and rods are,due to economic necessity, charged with several tons of bars at a singletime so that the desired uniformity can be achieved only by maintainingthe bars at the proper temperature for several hours, usually of theorder of five hours or more. Even with such a long residence time of thebars in the furnace, it is difficult to achieve ideal uniformity withrespect to both the temperature at which the bars are maintained and thetime during which the bars are maintained at the desired temperature andstill achieve economical production rates.

It is an object of the present invention to provide a method for themanufacture of stress relieved prestrengthened steel workpieces, such asbars, rods and the like, which overcomes the foregoing disadvantages,and it is a more specific object of the invention to provide a methodfor the manufacture of stress relieved prestrengthened steels havinghigh tensile strength, high yield strength and a low level of residualstresses.

It is another object of the present invention to provide a method forthe manufacture of stress relieved, prestrengthened elongate steelworkpieces, in which the method can be carried out quickly to producestress relieved products in an economical manner, and which can becarried out to provide improved uniformity in the heating operation,both from the standpoint of a single workpiece and from workpiece toworkpiece.

These and other objects and advantages of the invention will appear morefully hereinafter, and for purposes of illustration, but not oflimitation, an embodiment of the invention is shown in the accompanyingdrawings in which:

FIG. 1 is a graph illustrating the relationship of stress and strain forsteel workpieces before and after straightening;

FIG. 2 is a graph illustrating the effects of the time during which asteel is subjected to an elevated temperature stress relievingoperation, with tensile strength, yield strength and residual stresslevels;

FIG. 3 is a schematic illustration of a cold drawing operation;

FIG. 4 is a schematic illustration of a straightening operation using aLewis straightener;

FIGS. 5 and 6 illustrate the operation of a Medarttype straightener, and

FIG. 7 illustrates the preferred stress relieving operation carried outin accordance with the practice of this invention.

The concepts of the present invention reside in the discovery that thetensile and yield strengths of a steel can be maximized and the residualstress levels can be minimized where an elongate steel workpiece issubjected to the steps of cold working to prestrengthen the steel, thenstraightening the steel, followed by stress relieving at an elevatedtemperature where the stress relieving operation is performed by heatingthe workpiece rapidly to the desired temperature, generally within 10minutes. It has been found that elongate steel workpieces produced inthis manner provide greater product uniformity, improved machine-abilityand significantly lower levels of residual stresses.

As is illustrated in FIG. 1 of the drawing, which is a graph of stressversus strain, cold drawing of a steel workpiece provides a proportionalincrease in the stress as the strain on the workpiece is increased untilthe elastic limit of the steel is reached. At this point, the curvelevels off to provide a more gradual increase in stress as the strain,to which the bar is subjected, is increased. However, the straighteningoperation referred to above provides] a somewhat different effect. Asshown in FIG. 1 of the drawing, the relationship of stress with strainfor a steel workpiece which has been first subjected to cold drawing andthen subjected to straightening is somewhat different from that of asteel workpiece after drawing but before straightening. As is shown bythe lower curve in FIG. 1, the relationship of stress with strain for aworkpiece which has been cold drawn and then straightened is linearuntil the elastic limit is reached and then the stress increases moreslowly with increasing strain. Thus, the cold drawing operation operatesto increase the strength of the steel, but also to impart to it residualstresses. The straightening operation, on the other hand, serves torearrange or redistribute the residual stresses, but results in adecrease in the yield strength of the resulting workpiece. As is shownin FIG. 1, the yield strength, i.e. the stress at which the metal willdevelop a strain of 0.2% is, 2,. The value of the yield strength 2,, forthe workpiece, which has been subjected to cold drawing, is higher thanthe yield strength 2,, for the workpiece which has been subjected tocold drawing and then straightening.

The purpose of the stress relieving operation described above is tocause the yield strength to increase. There is shown in FIG. 2 of thedrawing a series of curves illustrating the effect of an elevatedtemperature stress relief operation on the properties of tensilestrength and yield strength as a factor of the logarithm of the timeduring which the heat treatment is carried out. For each property, thereare shown two curves, one for a temperature T (of the order of 600F),and the other for a temperature T; (of the order of 900F). As can beseen from these curves, both the tensile strength and the yield strengthof a steel workpiece which has been subjected to cold drawing and thenstraightening decrease as the elevated temperature stress reliefoperation is carried out, and the rate of decrease in the tensilestrength and the yield strength increases with the time of heattreatment. In the case of the residual stress level, however, the rateat which the residual stress level is decreasing is much faster. Inaccordance with the practice of this invention, the use of a rapiduniform heating step in the stress relieving operation serves to providegenerally the same reduction in the residual stress level, but serves tomaximize the tensile and yield strengths by limiting the time duringwhich the workpiece is subjected to the elevated temperature. In otherwords, by the use of a rapid uniform heating step the stress relievingoperation is carried out before a rapid decrease in the tensile strengthand yield strength of the material can occur.

In accordance with the practice of the present invention, an elongatesteel workpiece, which may be in the form of a rod, a bar or the likeworkpiece of repeating cross-section, is subjected to a cold workingoperation, preferably cold drawing, to effect a reduction in thecross-sectional area of the workpiece sufficient to provide a measurableincrease in the strength of the workpiece. The cold drawing operationcan be carried out in accordance with conventional techniques asillustrated in FIG. 3 of the drawing in which the elongate workpiece issimply advanced through a reduction die 12 to form a prestrengthenedworkpiece 14. As used herein, the term cold drawing refers to a drawingoperation carried out at a temperature below the lower criticaltemperature for the steel. After the cold drawing operation, theworkpiece is subjected to a straightening operation in which reversebending of the workpiece is performed, preferably by progressivelybending the workpiece through decreasing deflections to provide astraightened workpiece. For this purpose, use can be made of aconventional Lewis straightener which is schematically illustrated inFIG. 4 of the drawing, or use can also be made of a Medart straighteningmachine shown schematically in FIGS. 5 and 6 of the drawing. Suchstraighteners are themselves conventional and operate to straighten theworkpiece by bending the workpiece in decreasing amounts to produce astraightened workpiece.

In accordance with the preferred practice of the present invention,rapid heating of the workpiece to effect the strain relief operation ispreferably carried out by direct resistance heating. An electric currentis passed through the workpiece whereby the electrical resistance of theworkpiece to the flow of current causes rapid heating in the workpiece.The workpiece is preferably connected to a source of electric currentwith the connections being made at both ends so that the current flowscompletely through the bar. Since the current flows uniformly throughthe bar, the temperature of the bar increases uniformly, both axiallyand radially. In this way, the interior as well as the exterior of theworkpiece is heated simultaneously without introducing thermal strains.In the preferred practice of the invention, use can be made of a lowfrequency electric current or preferably direct current. It is thuspossible to heat a large workpiece in'a very short time simply with theuse of sufficiently large power levels.

One suitable means for heating the workpiece 14 by electrical resistanceis schematically illustrated in FIG. 7 of the drawing. As shown in thisFigure, electrical contacts 16 and 18 are positioned in contact with theworkpiece 14 whereby the flow of current between the two contacts 16 and18 passes through the entire length of the workpiece 14. It is generallypreferred to subject the workpiece 14, during the time of the heatingoperation, to tension to compensate for thermal expansion of theworkpiece and to avoid buckling in the workpiece while at an elevatedtemperature. The slight tension exerted on the workpiece during thestress relieving operation thus serves to preserve the straightness ofthe workpiece and effects no plastic deformation of the workpiece. Thestress relief operation is somewhat time and temperature dependent.

It has been found that when use is made of electrical resistance heatingto carry out the stress relieving operation on the workpiece, slightlyhigher temperatures, for example from 50 to 200F higher, are required toachieve the same mechanical property level as compared to typicalfurnace heating because of the relatively slow rates of change of theseproperties at a given temperature. However, because of this and becauseof the reduction in residual stress occurring much faster, substantiallylower levels of residual stress are achieved. It is essential to thepractice of this invention in producing a workpiece having a hightensile strength, a high yield strength and a low residual stress levelto carry out the heating operation uniformly and rapidly. In general,heating times required to achieve the desired temperature of less than10 minutes, and preferably 15 to seconds, are sufficient. Additionally,it may be desirable to maintain the workpiece at the desired temperaturefor one second to five minutes. The temperature, to which the workpieceis heated, is generally within the range of 500F to the lower criticaltemperature for the steel, and preferably from 500 to l300F. It isbelieved that the somewhat higher heating temperature employed inaccordance with the invention is due to the fact that the entire heatingoperation is performed so rapidly and uniformly as compared to theseveral hours heating employed in conventional furnaces.

It is known to the art to heat metal wire after it has been drawn tonormalize the strains in the wire. For example, reference can be made toYoung, US. Pat. No. 2,281,132. However, Young does not straighten thewire prior to normalizing the strains and, therefore, does not incur thedifficulty in decreased yield strength, contrary to the presentinvention.

The present invention is applicable to the treatment of a variety ofsteels, including carbon and low alloy steels, such as carbon steelscontaining 0.10 to 0.75% by weight carbon, and low alloy steelscontaining 0.10 to 0.75% by weight carbon.

The invention is particularly well suited in the production of rods andbars formed of AISl/SAE grade 1144, 4142 and 4l42I-I steels, as well asgrade 4142 and 4l42I-I steels which contain additives to improvemachinability. In general, such steels contain are provided for purposesof illustration, and not of limitation, of the practice of thisinvention.

EXAMPLE 1 Furnace (875F) Resistance (900F) Tensile Strength, 121,000127,000

psi

Yield Strength, 101,000 113,000 psi Elongation, l2 Reduction of Area, 3227 Hardness, Rc 23 23 All bars were then straightened and subsequentlytested for machinability on 1 inch RAN, 6-spindle National AcmeAutomatic Screw Machine. Each condition was run for an eight 'hour testat predetermined speeds and feeds to produce test parts.

The results of the tests showed that the resistance stress relievedmaterial was clearly superior in the two parameters observed: drillingcharacteristics and part growth. While it was necessary to change thedrills three times during the eight hour run with the furnace treatedmaterial, the resistance heated steel ran the entire eight hours withthe same set of drills. Similarly, the part growth (which is related totoo] wear) for the resistance heated steel was only 0.0008 inches ascompared to 0.0015 inches for the furnace treated material.

A further interesting observation was made during the course of thistest concerning the uniformity of the two conditions of material.Extensive sampling and hardness testing of both conditions showed thefurnace treated material to have a hardness spread of 6 R0 'points,whereas the spread in hardness for the resistance treated material wasonly 4 Re points. Apparently, the uniformity of the resistance heatedsteel is significantly better than the furnace treated equivalent.

EXAMPLE 2 Another group of 1 144 steel bars, all from the same heat, wasprocessed in the same manner as described in Example 1 to provide thesame mechanical properties. The furnace temperature in this Example was850F and the temperature to which the bars were heated by resistance was1030F. The two conditions werethen tested relative to one another onwhat has historically been a critical machining application involvingvery light surface removal cuts with a form tool as well as a longcenter drilling. The critical parameter was the life of the form tool onthe automatic screw machine as measured by the number of parts producedat the time of failure.

The results of this test showed that the resistance stress relievedmaterial produced 1290 parts prior to form tool failure, whereas thefurnace treated material produced only 970 parts. It may be concluded,therefore, that the resistance stress relieving treatment resulted in a33% increase in the life of the form tool in this operation.

EXAMPLE 3 A third group of 1144 steel bars, all from the same heat, wasagain produced in the same manner as previously described, with afurnace temperature of 900F and a resistance heating temperature of1050F, to obtain the same mechanical properties. The two treatmentconditions were then tested on a Pratt and Whitney lead screw cutting,machine which was set up to cut a 1 inch 5 Acme screw thread.

Distortion due to residual stresses in prestrengthened cold finishedsteel bars is a major problem for manufacturers of lead screws. Themeasure of this distortion is the lead error in one foot. A lead errorof less than 0.0003 inches in one foot is generally considered to beexcellent, and is usually obtained by grinding after a rough and finishcut.

The. following lead error measurements were taken after one full depthcut in one pass for the two stress relieved conditions:

a. Furnace stress relieved0.0012 inch in one foot.

b. Resistance stress relieved 0.00005 inch in one foot.

These results are phenomenal in light of the fact that the twoconditions were both produced to 125,000 psi tensile strength and thatthe only difference between the two is the method of heating used toproduce them. The results indicate that the resistance stress relievedbars had over an order of magnitude lower residual stress level than thefurnace treated bars.

It will be understood that various changes and modifications in detailsof procedure, operation and use may be made without departing from thespirit of the invention, especially as defined by the following claims.

We claim:

1. A method for the manufacture of stress relieved, pre-strengthenedelongated steel workpieces comprising the steps of cold working aworkpiece formed of a carbon or low alloy steel to pre-strengthen theworkpiece, straightening the workpiece'by reverse bending and rapidlyheating the workpiece to a temperature within the range of 500F to thecritical temperature of the steel in less than 10 minutes whereby theworkpiece is stress relieved such that high levels of mechanicalproperties are maintained with low levels of residual stress.

2. A method as defined in claim 1 wherein the workpiece is heated bypassing an electrical current through the workpiece.

3. A method as defined in claim 1 wherein the workpiece is heated to atemperature within the range of 500 to 1300 F.

4. A method as defined in claim 1 wherein the workpiece is heated to thedesired temperature within 1 second to 5 minutes.

5. A method as defined in claim 1 wherein the workpiece is straightenedby advancing the workpiece through a Lewis straightener.

6. A method as defined in claim 1 wherein the workpiece is straightenedby advancing the workpiece through a Medart straightener.

7. A method as defined in claim 1 wherein the steel is a carbon or lowalloy steel having a carbon content within the range of 0.10 to 0.75% byweight.

8. A method as defined in claim 1 wherein the steel is a AlSI/SAE 1 M4steel.

9. A method as defined in claim 1 wherein the workpiece is drawn toreduce the cross-section of the workpiece and to increase the tensilestrength of the workpiece.

10. A method as defined in claim 1 wherein the workpiece is subjected tocold drawing.

11. A method as defined in claim 1 wherein the cold working is a colddrawing operation carried out at a temperature below the lower criticaltemperature for the steel.

12. A method as defined in claim 1 wherein the workpiece is heated at arate substantially uniform over the cross-section thereof.

13. A method as defined in claim 1 wherein the straightening is carriedout by passing the workpiece between a plurality of rollers to bend andstraighten the workpiece.

14. A pre-strengthened, stress relieved steel workpiece formed of acarbon or low alloy steel prepared by the method of claim 1.

15. A steel as defined in claim 14 wherein the steel contains 0.10 to0.75% carbon by weight.

16. A steel as defined in claim 14 wherein the steel is a AlSI/SAE l 144steel.

17. A steel as defined in claim 14 wherein the steel contains 0.10 to0.75% by weight C, 0.50 to 1.5% by weight Mn, 0.01 to 0.07% by weight P,0.01 to 0.50% by weight S, 0 to 1.5% by weight Cr, 0 to 0.50% by weightMo and 0.10 to 0.80% by weight Si, with the remainder being iron and itsusual impurities.

18. A pre-strengthened, stress relieved steel workpiece formed of acarbon or low alloy steel prepared by the method of claim 2.

19. A method for the manufacture of stress relieved, prestrengthenedelongate steel workpieces comprising the steps of cold drawing aworkpiece formed of a carbon or low alloy steel at a temperature belowthe lower critical temperature for the steel to prestrengthen theworkpiece, passing the workpiece between a plurality of rollers to bendand straighten the workpiece, and passing an electric current throughthe workpiece to heat the workpiece substantially uniformly across thecross-section thereof to a temperature within the range of 500F to thelower critical temperature of the steel in less than ten minutes wherebythe workpiece is stress relieved such that high levels of residualstress.

20. A method for the manufacture of stress relieved, prestrengthenedworkpieces comprising the steps of cold working the workpiece formed ofa carbon or low alloy steel to prestrengthen the workpiece, and rapidlyheating the workpiece to a temperature within the range of 500F to thelower critical temperature of the steel in less than ten minutes wherebythe workpiece is stress relieved such that high levels of mechanicalproperties are maintained with low levels of residual stress.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 5 3,908 43l DatedSeptember 30, 1975 fls) Samuel H. Jones and James M. McNeany' It iscertified that error appears in the aboveidentified patent nd that saidLetters Patent are hereby corrected as shown below:

a In column 8, line 25, after "levels", please insert of mechanicalproperties are maintained with low levels Signed and Sealed thisfifteenth D f June 1976 [SEAL] AMEN.

RUTH C. MASON C. MARSHALL DANN Ammng Officer (mnmissimwr of Palems andTrademarks

1. A METHOD FOR THE MANUFACTURE OF STRESS RELIEVED, PRESTRENGTHENEDELONGATED STEEL WORKPIECES COMPRISING THE STEPS OF COLD WORKING AWORKPIECE FORMED OF A CARBON OR LOW ALLOY STEEL TO PRE-STRENGTHEN THEORKPIECE, STRAIGHTENING THE WORKPIECE BY REVERSE BENDING AND RAPIDLYHEATING THE WORKPIECE TO A TEMPERATURE WITHIN THE RANGE OF 500*F TO THECRITICAL TEMPERATURE OF THE STEEL IN LESS THAN 10 MINUTES WHEREBY THEWORK-
 2. A method as defined in claim 1 wherein the workpiece is heatedby passing an electrical current through the workpiece.
 3. A method asdefined in claim 1 wherein the workpiece is heated to a temperaturewithin the range of 500* to 1300 *F.
 4. A method as defined in claim 1wherein the workpiece is heated to the desired temperature within 1second to 5 minutes.
 5. A method as defined in claim 1 wherein theworkpiece is straightened by advancing the workpiece through a Lewisstraightener.
 6. A method as defined in claim 1 wherein the workpiece isstraightened by advancing the workpiece through a Medart straightener.7. A method as defined in claim 1 wherein the steel is a carbon or lowalloy steel having a carbon content within the range of 0.10 to 0.75% byweight.
 8. A method as defined in claim 1 wherein the steel is aAISI/SAE 1144 steel.
 9. A method as defined in claim 1 wherein theworkpiece is drawn to reduce the cross-section of the workpiece and toincrease the tensile strength of the workpiece.
 10. A method as definedin claim 1 wherein the workpiece is subjected to cold drawing.
 11. Amethod as defined in claim 1 wherein the cold working is a cold drawingoperation carried out at a temperature below the lower criticaltemperature for the steel.
 12. A method as defined in claim 1 whereinthe workpiece is heated at a rate substantially uniform over thecross-section thereof.
 13. A method as defined in claim 1 wherein thestraightening is carried out by passing the workpiece between aplurality of rollers to bend and straighten the workpiece.
 14. Apre-strengthened, stress relieved steel workpiece formed of a carbon orlow alloy steel prepared by the method of claim
 1. 15. A steel asdefined in claim 14 wherein the steel contains 0.10 to 0.75% carbon byweight.
 16. A steel as defined in claim 14 wherein the steel is aAISI/SAE 1144 steel.
 17. A steel as defined in claim 14 wherein thesteel contains 0.10 to 0.75% by weight C, 0.50 to 1.5% by weight Mn,0.01 to 0.07% by weight P, 0.01 to 0.50% by weight S, 0 to 1.5% byweight Cr, 0 to 0.50% by weight Mo and 0.10 to 0.80% by weight Si, withthe remainder being iron and its usual impurities.
 18. Apre-strengthened, stress relieved steel workpiece formed of a carbon orlow alloy steel prepared by the method of claim
 2. 19. A method for themanufacture of stress relieved, prestrengthened elongate steelworkpiEces comprising the steps of cold drawing a workpiece formed of acarbon or low alloy steel at a temperature below the lower criticaltemperature for the steel to prestrengthen the workpiece, passing theworkpiece between a plurality of rollers to bend and straighten theworkpiece, and passing an electric current through the workpiece to heatthe workpiece substantially uniformly across the cross-section thereofto a temperature within the range of 500*F to the lower criticaltemperature of the steel in less than ten minutes whereby the workpieceis stress relieved such that high levels of residual stress.
 20. Amethod for the manufacture of stress relieved, prestrengthenedworkpieces comprising the steps of cold working the workpiece formed ofa carbon or low alloy steel to prestrengthen the workpiece, and rapidlyheating the workpiece to a temperature within the range of 500*F to thelower critical temperature of the steel in less than ten minutes wherebythe workpiece is stress relieved such that high levels of mechanicalproperties are maintained with low levels of residual stress.